1. Field of the Invention
The present invention relates to antennas, and more particularly relates to a boxed-in slot antenna having a folded, space-saving configuration, which can be employed in space-critical applications, such as a laptop computer.
2. Brief Description of the Prior Art
There is increased interest in enabling laptop computers and other portable electronic devices to interface with a wireless local area network (WLAN). WLANs may operate under a number of standards, for example, the so-called "Bluetooth" standard. In such systems, an antenna is required to send and receive data via radio frequency (RF) communications.
In portable electronic devices, space is typically at a premium. Accordingly, it is desirable to minimize space to be occupied by an antenna in such devices. One prior art approach to providing an RF antenna which takes up minimal space is disclosed in World Intellectual Property Organization (WIPO) international publication number WO 95/06338 published on Mar. 2, 1995. In this publication, a folded monopole antenna is discussed. The folding of the monopole reduces its height so as to enable it to fit into small areas. However, the folding of the monopole has undesirable effects on the electrical match, frequency bandwidth and electromagnetic fields, requiring the introduction of a shunt inductance between the monopole and the ground plane.
Slot antennas are known in the prior art, and are useful for low-profile or flush installations, such as in high-speed aircraft. A traditional slot antenna is described in the book Antennas by John D. Kraus, at pages 624-632 (Second Edition, McGraw-Hill 1988). FIG. 1 shows a prior art slot antenna, designated generally as 10. A conductive ground plane 12, typically metallic, is formed with a slot 14. The slot has a length, L, which is typically equal to half of the electric wavelength .lambda..sub.e. The slot 14 typically also has a width, w, which is much less than the wavelength. Such an antenna will radiate equally from both sides of the ground plane 12. It is typically fed by a coaxial cable 16, which can be attached at an off-center feed point in order to obtain a 50 Ohm antenna impedance so as to match the characteristic impedance, typically 50 Ohms, of coaxial cables.
In some applications, it is desirable to have a slot antenna which radiates in only one direction. This can be achieved with a fairly large conductive ground plane, with one side of the slot boxed-in, as shown in FIG. 2. This type of structure is also discussed in the aforementioned Kraus reference book. The prior-art boxed-in slot antenna of FIG. 2 is designated generally as 20. The antenna 20 of FIG. 2 is also formed with a conductive ground plane 22, and with a slot having dimensions L,w as before. The slot is designated as 24. A box structure 26 is used to box-in the slot 24, and typically extends a depth, h, below the surface of the conductive ground plane 22. The distance h is typically one-quarter of the waveguide wavelength .lambda..sub.g. The box structure 26 blocks radiation in the rearward direction in FIG. 2, so that radiation in the forward direction is enhanced; further, it doubles the radiation resistance of the original slot antenna 10. Feed can be via a coaxial cable 28. The original slot antenna 10 is not appropriate for use in a handheld electronic device or a laptop computer because of the radiation in both directions, while the prior art boxed-in slot antenna of FIG. 2 is also unsuitable, as the distance h must be so large that the antenna occupies an unacceptably large space. Note that the Kraus reference uses "d" for "h"; the "h" terminology is used herein to avoid confusion with a "d" parameter referred to below with respect to the present invention.
It will be appreciated that the prior art folded monopole approach of the aforementioned WIPO publication results in unadvantageous changes to the electrical match, frequency bandwidth and electromagnetic fields, necessitating the introduction of a shunt inductance. Further, the slot antennas discussed immediately above are unsuitable due to either bi-directional radiation or excessive size.
In view of the foregoing, there is a need in the prior art for a compact antenna suitable for use in laptop computers and other portable electronic devices. There is the need for such an antenna which takes up minimal space, can be easily fabricated, and has desirable electric characteristics.